Underground cables and pipelines are typically emplaced by laying the cable or pipeline in a prepared trench and subsequently backfilling the trench.
Some cables and pipelines are susceptible to damage from stones, rocks, or other hard objects in the backfill material. For example, optical fiber communications cables are considered particularly susceptible to damage in this manner, as are polymeric or plastic pipelines. Also steel pipes are increasingly provided with protective polymeric coatings, which must be protected from penetration or damage by hard objects.
Consequently, in the laying of cables and pipelines it is increasingly sought to backfill the trench with fill material that is relatively free of stones or other hard objects. One way to achieve this is to backfill the trench with sand or other suitable fill material brought from a remote source of sand or rock-free soil. This approach is relatively expensive and time-consuming. Further, where steel pipe is protectively padded with a layer of sand, the filled trench tends to collect standing water in the porous sand fill, leading to premature corrosion of the pipe. Also, the use of a fill material that is different from the surrounding soil results in a loss of cathodic protection, which also leads to premature corrosion of steel pipe.
The alternative is to screen the excavated material dug from the trench, to remove stones and other foreign objects, and return the screened material to the trench. Several machines, known as padding machines, have been disclosed in the prior art for this purpose.
For example, U.S. Pat. No. 2,857,691 to Curran discloses a tracked vehicle having a vertically swingable boom that extends laterally over a trench. The boom includes a tube having an enclosed auger. At the far end of the boom from the vehicle is a rotating head which scoops up soil from alongside the trench, screens the soil, and transmits it to the auger, which conveys the screened soil along the tube and into the trench through openings in the tube. The Curran apparatus is particularly designed for use with a vehicle that is driven along the opposite side of a trench from the pile of soil that was removed from the trench and which extends alongside the trench. One of the major disadvantages of the Curran device is that since the rotating head is attached with boom extending to the side of the tracked vehicle across the ditch, the operation of the rotating head on the far side of the ditch is difficult or impossible to control. Another major disadvantage is that the device occupies both sides of the ditch, causing increased traffic congestion along the narrow right of way of the pipeline.
U.S. Pat. No. 4,633,602, to Layh et al., is an important advance in the pipeline padding art. Layh et al. discloses a pipeline padding machine built on a vehicle frame that operates on one side of the ditch in the excavated material pile. Layh et al. shows the use of a pipeline padding machine having a pivotally mounted gathering head for collecting excavated material and an elevator for moving collected excavated material upwardly and rearwardly to dump the material onto a fiat separator screen. Fine material falls through the separator screen onto a lateral conveyor for transporting the fine padding material to the ditch. The angle or tilt of the separator screen can be adjusted to compensate for the grade of upward and downward slopes of the pipeline environment and the type of excavated material to obtain a sufficient amount of padding material separated from the excavated material. The rough material is placed behind the padding machine. The separator screen can be vibrated to increase the amount of fines passed through the separator. This device does not provide for screening during the initial conveying nor for attachment to vehicles, such as loaders and bulldozers.
U.S. Pat. Nos. 5,120,433 and 5,195,260, issued to Mark Osadchuk, disclose significantly improved pipeline padding machines of the general type disclosed in U.S. Pat. No. 4,633,602 to Layh et al. However, the type of padding machines disclosed in the Layh et al. and Osadchuk patents are generally large machines, which are intended and useful primarily for long-distance pipe laying operations in open country, where rights of way are relatively wide and where the terrain is not excessively rugged.
U.S. Pat. Nos. 5,097,610 and 5,261,171 issued to Bishop disclose a pipeline padding machine attachment. The Bishop padding machine attachment is removably attached to one end of a vehicle, such as a bucket loader, bulldozer, or other base machine, whereby the attachment moves along the path of travel of the vehicle. The padding machine attachment picks up at least a portion of a pile of excavated material along side the ditch and processes the excavated material into padding material for placement into the ditch for padding a cable or pipeline.
While the Layh et al., Osadchuk, and Bishop devices are suitable for their respective intended purposes, it is desirable to substantially increase the ability of a padding machine to separate fine padding material from excavated material. There has been a particularly long-felt need to increase this ability for small pipeline padding machines or pipeline padding machine attachment devices, which have the advantage of versatility and lower cost, but suffer from the disadvantage of inadequate padding material output for larger padding operations.